This invention relates to an overflow line for the reactor vessel of a liquid-metal-cooled nuclear reactor. In more detail, the invention relates to an overflow line for the reactor vessel of a sodium-cooled nuclear reactor employing hot leg pumps including means for establishing and maintaining a continuous bleed flow of coolant through the overflow line.
The sodium level within the reactor vessel of a sodium-cooled nuclear reactor employing hot leg pumps is maintained by an overflow/makeup system with constant overflow during steady-state operation. An overflow line is installed in the reactor to limit the liquid level in the reactor vessel and a makeup system is provided to accommodate sodium volumetric changes due to temperature changes as the Reactor/Plant power level changes. Following plant trips wherein control rods are inserted and the main sodium circulating pumps are shut off, the sodium level in the reactor falls. This is due to: (a) The level in the hot leg free surface pumps rises; i.e., the pump drawdown vanishes. Refilling the pump tanks is essentially a shift in sodium inventory from the reactor vessel to the pump tank. (b) The shrinkage of sodium in the primary system as the temperature drops in the primary system -- the ratio of the heat transfer rate in the intermediate heat exchanger to the decay power in the reactor is greater than one.
The drop in sodium level in the reactor vessel (which may be about two feet) interrupts the flow through the overflow line. By the time the flow is reestablished -- about an hour -- the sodium which first overflows the reactor vessel is considerably lower in temperature -- on the order of 200.degree. -- than is the overflow line itself and the nozzle at the overflow tank which have remained hot. The sodium in the reactor vessel is continuously circulated through the intermediate heat exchanger after shutdown of the reactor -- and thereby cooled -- since the pumps continue to operate at shutdown speed. The flow of the cooler sodium through the hot overflow line and nozzle at the overflow tank which occurs when the reactor is restarted represents an undesirable thermal shock to the hardware in question.